The present invention relates generally to a temperature measurement system for use in integrated circuits and more particularly to a temperature measurement system based on current mode sigma-delta modulation for use within dynamic random access memory (DRAM) devices.
Temperature sensors are used within integrated circuits, for example, to protect against overcurrent damage, to compensate for cross sensitivity of other sensors, to reduce errors caused by self-heating, and to provide process data input, among others. Increasingly, complimentary-metal-oxide-semiconductor (CMOS) devices are used as temperature sensors due to the ease of incorporating these devices into the integrated circuit.
FIG. 7 illustrates a temperature measurement system according to the prior art. The temperature measurement system includes a temperature sensor 100, a bandgap voltage reference circuit 102, a sigma-delta converter 104, a counter 106, and a controller 108, among others. The temperature measurement system is used to convert an analog temperature reading, as produced by temperature sensor 100, into a digital output.
The forward voltage of a diode decreases linearly with temperature. Utilizing this characteristic, methods and circuits to derive temperature and reference signals from CMOS devices have been developed and are well known. Thus, a detailed discussion of such methods and circuits is omitted herein. Temperature sensor 100 may be a CMOS device comprised of p-channel and/or n-channel transistors. As seen in FIG. 7, temperature sensor 100 produces a temperature dependent current (ITEMP) that is provided to sigma-delta converter 104.
Reference circuit 102 is comprised of precision analog components and produces a reference current (IREF) and a reference voltage (VREF). The reference current (IREF) and the reference voltage (VREF) may also be referred to as the bandgap reference current (IBGref) and bandgap reference voltage (VBGref), respectively. Both IREF and VREF are temperature independent. Although capable of producing a temperature independent current and a temperature independent voltage, the precision analog components used by reference circuit 102 are costly and require band-gap type tuning. As seen in FIG. 7, IREF and VREF are provided to sigma-delta converter 104.
Sigma-delta converter 104 uses ITEMP, IREF, and VREF to produce a bitstream that is provided to counter 106. Counter 106 uses the bitstream to produce a digital output representing the temperature sensed by temperature sensor 100. Controller 108 controls the overall operation of the temperature measurement system. For example, controller 108 issues “power on”, “reset”, and “enable” signals (among others) to the other components of the temperature measurement system.
FIG. 8 illustrates a simplified circuit diagram of the prior art sigma-delta converter 104 of FIG. 7. Sigma-delta converter 104 includes switches 120, 122, a capacitor 124, an op-amp 126, a comparator 128, and a flip-flop register 130. In operation, Switch 120 is responsive to a feedback loop from the output of flip-flop register 130. ITEMP (e.g., from temperature sensor 100 as shown in FIG. 7) is added to IREF when switch 120 is closed. The combined signal is then fed to an integrator which, as shown in FIG. 8, is formed by the combination of op-amp 126, capacitor 124, and switch 122. Switch 122 is responsive to a reset signal. If switch 122 is in its open state (and switch 120 is in its closed state), ITEMP and IREF cause a voltage to develop across capacitor 124. This voltage also develops at the output of op-amp 126, which is fed to the non-inverting input of comparator 126. The output of the op-amp 126 is compared to a reference signal (e.g., ground) by comparator 128 and the output of the comparator 128 is fed to an input of flip-flop register 130. The output of the flip-flop register 130 carries a bitstream which, as discussed above, is fed back to switch 120 and also fed to a counter (not shown in FIG. 8). The counter (e.g., counter 106 as shown in FIG. 7) tracks the number of “1” decisions made by comparator 128 in a predetermined time period and produces the digital output representing the temperature sensed by the temperature sensor 100.
The prior art temperature measurement system's resolution, power consumption, and need for band-gap type tuning, however, are not adequate for certain integrated circuit applications. Additionally, the sigma-delta converter's 104 use of IREF and VREF fails to insure adequate operation at low voltages (e.g., 1.2 V and below). With respect to resolution, for example, the output of comparator 122 is fed to counter 106 as discussed above. The counter 106 is activated for predetermined time period (e.g., 100 cycles of a self-generated clock signal). After this predetermined time period expires, the counter's 106 output is read and the sensing operation is completed. For a typical prior art temperature measurement system operated at a temperature range between approximately −40° C. and 110° C., the counter 106 range is approximately 15 for every 100 times a sample of the comparator output is taken.
Accordingly, a need exists for a temperature measurement system which overcomes these problems and which overcomes other limitations inherent in prior art.